I propose a study of the genetic variability and intraspecific divergence of natural populations of small mammals on the Channel Islands off the coast of California. The overall objective of this work is to detect microevolutionary changes as they occur in these island "natural laboratories." All the islands have evolved endemic subspecies of the deer mouse, Peromyscus maniculatus, and two support endemic subspecies of the harvest mouse, Reithrodontomys megalotis. The immediate objectives are to determine the present levels of genic heterozygosity within subspecies and the genetic divergence between subspecies. The potential significance of these data is that: 1) they will provide a baseline from which to assess further genetic changes; 2) they may be related to the periods of goegraphical isolation and thus contribute to our understanding of rates of evolution; 3) they will add new information on which to judge the controversy between the selectionist view of evolution and the neutral mutation hypothesis. Genic heterozygosity will be estimated by starch gel electrophoresis of blood and tissue samples from animals live trapped on the islands. The genetic control of the electrophoretic variants will be ascertained by crossing animals in the laboratory. The genetic distance between populations will be determined using the method developed by Nei.